1. Technology Field
The invention generally relates to a signal generating system, and more particularly, to a frequency generating system.
2. Description of Related Art
Frequency generating system plays a very important role in the communication engineering field. Particularly, phase-locked loop (PLL) for generating frequency is broadly adopted in modulation, demodulation, frequency multiplication, frequency synthesis, carrier synchronization, bit synchronization, etc. A PLL is a feedback loop, in which the frequency and phase of an output clock signal are locked at the frequency and phase of an input reference clock signal by using a feedback signal. Thus, in radio communication, if carrier frequency drift occurs during signal transmission, the local oscillation frequency of a receiver can drift along with the carrier frequency drift when a PLL is adopted in the receiver, so that the purpose of phase locking is achieved.
A PLL can be categorized as a PLL with a single-path architecture or a PLL with a dual-path architecture according to the control mechanism adopted by a voltage-controlled oscillator (VCO) inside the PLL. In an application of a high-frequency communication chip, a PLL with the dual-path architecture requires a small layout area. Accordingly, the fabrication cost of the high-frequency communication chip can be effectively reduced. Generally speaking, the capacitance-voltage characteristic curve of a varactor in a VCO of a dual-path PLL may drift when there is process variation or temperature change. This drifting phenomenon makes the frequency gain of the VCO inconsistent to an up signal and a down signal of the PLL. As a result, the up signal and the down signal present an asymmetrical state after they are converted by the VCO. Accordingly, a phase-locked clock generated by the PLL carries spur noise even after it is stabilized. Namely, the signal quality is low.
Nothing herein should be construed as an admission of knowledge in the prior art of any portion of the present invention. Furthermore, citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention, or that any reference forms a part of the common general knowledge in the art.